Semiconductor device and method for fabricating the same

ABSTRACT

A method for fabricating semiconductor device includes the steps of: forming an inter-metal dielectric (IMD) layer on a substrate; forming a metal interconnection in the IMD layer; forming a bottom electrode layer on the IMD layer, wherein the bottom electrode layer comprises a gradient concentration; forming a free layer on the bottom electrode layer; forming a top electrode layer on the free layer; and patterning the t op electrode layer, the free layer, and the bottom electrode layer to form a magnetic tunneling junction (MTJ).

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates to a semiconductor device and method forfabricating the same, and more particularly to a magnetoresistive randomaccess memory (MRAM) and method for fabricating the same.

2. Description of the Prior Art

Magnetoresistance (MR) effect has been known as a kind of effect causedby altering the resistance of a material through variation of outsidemagnetic field. The physical definition of such effect is defined as avariation in resistance obtained by dividing a difference in resistanceunder no magnetic interference by the original resistance. Currently, MReffect has been successfully utilized in production of hard disksthereby having important commercial values. Moreover, thecharacterization of utilizing GMR materials to generate differentresistance under different magnetized states could also be used tofabricate MRAM devices, which typically has the advantage of keepingstored data even when the device is not connected to an electricalsource.

The aforementioned MR effect has also been used in magnetic field sensorareas including but not limited to for example electronic compasscomponents used in global positioning system (GPS) of cellular phonesfor providing information regarding moving location to users. Currently,various magnetic field sensor technologies such as anisotropicmagnetoresistance (AMR) sensors, GMR sensors, magnetic tunnelingjunction (MTJ) sensors have been widely developed in the market.Nevertheless, most of these products still pose numerous shortcomingssuch as high chip area, high cost, high power consumption, limitedsensibility, and easily affected by temperature variation and how tocome up with an improved device to resolve these issues has become animportant task in this field.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, a method forfabricating semiconductor device includes the steps of: forming aninter-metal dielectric (IMD) layer on a substrate; forming a metalinterconnection in the IMD layer; forming a bottom electrode layer onthe IMD layer, wherein the bottom electrode layer comprises a gradientconcentration; forming a free layer on the bottom electrode layer;forming a top electrode layer on the free layer; and patterning the topelectrode layer, the free layer, and the bottom electrode layer to forma magnetic tunneling junction (MTJ).

According to another aspect of the present invention, a semiconductordevice includes a magnetic tunneling junction (MTJ) on a substrate, inwhich the MTJ further includes: a bottom electrode layer, wherein thebottom electrode layer comprises a gradient concentration; a free layeron the bottom electrode layer; and a top electrode layer on the freelayer.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-6 illustrate a method for fabricating a MRAM device according toan embodiment of the present invention.

FIGS. 7-8 illustrate a method for fabricating a MRAM device according toan embodiment of the present invention.

DETAILED DESCRIPTION

Referring to FIGS. 1-6, FIGS. 1-6 illustrate a method for fabricating asemiconductor device, or more specifically a MRAM device according to anembodiment of the present invention. As shown in FIG. 1, a substrate 12made of semiconductor material is first provided, in which thesemiconductor material could be selected from the group consisting ofsilicon (Si), germanium (Ge), Si—Ge compounds, silicon carbide (SiC),and gallium arsenide (GaAs), and a MTJ region 14 and a logic region 16are defined on the substrate 12.

Active devices such as metal-oxide semiconductor (MOS) transistors,passive devices, conductive layers, and interlayer dielectric (ILD)layer 18 could also be formed on top of the substrate 12. Morespecifically, planar MOS transistors or non-planar (such as FinFETs) MOStransistors could be formed on the substrate 12, in which the MOStransistors could include transistor elements such as gate structures(for example metal gates) and source/drain region, spacer, epitaxiallayer, and contact etch stop layer (CESL). The ILD layer 18 could beformed on the substrate 12 to cover the MOS transistors, and a pluralityof contact plugs could be formed in the ILD layer 18 to electricallyconnect to the gate structure and/or source/drain region of MOStransistors. Since the fabrication of planar or non-planar transistorsand ILD layer is well known to those skilled in the art, the details ofwhich are not explained herein for the sake of brevity.

Next, metal interconnect structures 20, 22 are sequentially formed onthe ILD layer 18 on the MTJ region 14 and the edge region 16 toelectrically connect the aforementioned contact plugs, in which themetal interconnect structure 20 includes an inter-metal dielectric (IMD)layer 24 and metal interconnections 26 embedded in the IMD layer 24, andthe metal interconnect structure 22 includes a stop layer 28, an IMDlayer 30, and metal interconnection 32 embedded in the stop layer 28 andthe IMD layer 30.

In this embodiment, each of the metal interconnections 26 from the metalinterconnect structure 20 preferably includes a trench conductor 46 andeach of the metal interconnection 32 from the metal interconnectstructure 22 on the MTJ region 14 includes a via conductor. Preferably,each of the metal interconnections 26, 32 from the metal interconnectstructures 20, 22 could be embedded within the IMD layers 24, 30 and/orstop layer 28 according to a single damascene process or dual damasceneprocess. For instance, each of the metal interconnections 26, 32 couldfurther includes a barrier layer 34 and a metal layer 36, in which thebarrier layer 34 could be selected from the group consisting of titanium(Ti), titanium nitride (TiN), tantalum (Ta), and tantalum nitride (TaN)and the metal layer 36 could be selected from the group consisting oftungsten (W), copper (Cu), aluminum (Al), titanium aluminide (TiAl), andcobalt tungsten phosphide (CoWP). Since single damascene process anddual damascene process are well known to those skilled in the art, thedetails of which are not explained herein for the sake of brevity. Inthis embodiment, the metal layers 36 are preferably made of copper, theIMD layers 24, 30 are preferably made of silicon oxide, and the stoplayer 28 is preferably made of nitrogen doped carbide (NDC), siliconnitride, silicon carbon nitride (SiCN), or combination thereof.

Next, a bottom electrode layer 44 and a patterned mask 38 such as apatterned resist are formed on the IMD layer 30, in which the patternedmask 38 preferably includes an opening 40 exposing the top surface ofthe bottom electrode layer 44. In this embodiment, the bottom electrode44 preferably includes conductive material such as TaN, but could alsoinclude other material including but not limited to for example Ta, Pt,Cu, Au, Al, or combination thereof.

It should be noted that the bottom electrode layer 44 preferablyincludes a concentration gradient such that the concentrationdistribution is not uniform throughout the entire bottom electrode layer44 and the concentration distribution could gradually increase upward,decrease upward, increase downward, or decrease downward depending onthe material being used. In this embodiment the bottom electrode layer44 is preferably made of TaN and in such instance, the concentration ofnitrogen preferably increases from a bottom surface of the bottomelectrode layer 44 to a top surface of the bottom electrode layer 44while the concentration of tantalum decreases from a bottom surface ofthe bottom electrode layer 44 to a top surface of the bottom electrodelayer 44. In other words, the region or area in the layer 44 immediatelyadjacent to the bottom surface of the bottom electrode layer 44 orimmediate adjacent to the junction between the bottom electrode layer 44and the metal interconnection 32 preferably includes higherconcentration of tantalum atoms and/or lower concentration of nitrogenatoms, whereas the region or area in the layer 44 immediately adjacentto the top surface of the bottom electrode layer 44 preferably includeshigher concentration of nitrogen atoms and/or lower concentration oftantalum atoms. Specifically, a ratio of nitrogen to tantalum closest toor at a top surface of the bottom electrode layer 44 is between 1.53 to1.87 and a ratio of nitrogen to tantalum closest to or at a bottomsurface of the bottom electrode layer 44 is between 0.18 to 0.22.

Next, as shown in FIG. 2, an etching process is conducted by using thepatterned mask 38 as mask to remove part of the bottom electrode layer44 and part of the IMD layer 30 to form a trench 42 serving as analignment mark 54, and the patterned mask 38 is removed thereafter. Itshould be noted that even though the bottom surface of the trench 42 iseven with a bottom surface of the IMD layer 30 in this embodiment,according to another embodiment of the present invention, the bottomsurface of the trench 42 could also be slightly higher than the bottomsurface of the IMD layer 30, slightly lower than the bottom surface ofthe IMD layer 30 and extended into part of the stop layer 28, which areall within the scope of the present invention.

Next, as shown in FIG. 3, a pinned layer 46, a barrier layer 48, a freelayer 50, and a top electrode layer 52 are sequentially formed on thebottom electrode layer 44 and filled into the trench 42.

Next, as shown in FIG. 4, a photo-etching process is conducted topattern the top electrode layer 52, the free layer 50, the barrier layer48, the pinned layer 46, and the bottom electrode layer 44 to form a MTJ62 and an alignment mark 56 adjacent to the MTJ 62, in which thealignment mark 56 could be used to facilitate the formation of metalinterconnections on the logic region 16 in later process. In thisembodiment, the pinned layer 46 could be made of antiferromagnetic (AFM)material including but not limited to for example ferromanganese (FeMn),platinum manganese (PtMn), iridium manganese (IrMn), nickel oxide (NiO),or combination thereof, in which the pinned layer 46 is formed to fix orlimit the direction of magnetic moment of adjacent layers. The barrierlayer 48 could include oxide containing insulating material such as butnot limited to for example aluminum oxide (AlO_(x)) or magnesium oxide(MgO). The free layer 50 could be made of ferromagnetic materialincluding but not limited to for example iron, cobalt, nickel, or alloysthereof such as cobalt-iron-boron (CoFeB), in which the magnetizeddirection of the free layer 50 could be altered freely depending on theinfluence of outside magnetic field. The top electrode layer 52 and thebottom electrode layer 44 could be made of same or different conductivematerials while the two layers 52, 44 could all be selected from thegroup consisting of Ta, Pt, Cu, Au, and Al.

It should be noted that an ion beam etching (IBE) process is preferablyconducted to remove part of the top electrode layer 52, part of the freelayer 50, part of the barrier layer 48, part of the pined layer 46, partof the bottom electrode layer 44, and part of the IMD layer 30 to formthe MTJ 62. Due to the characteristics of the IBE process, the topsurface of the remaining IMD layer 30 is slightly lower than the topsurface of the metal interconnection 32 after the IBE process and thetop surface of the IMD layer 30 also reveals a curve or an arc.

It should also be noted that when the IBE process is conducted to removepart of the IMD layer 30, part of the metal interconnection 32 isremoved at the same time so that a first slanted sidewall 64 and asecond slanted sidewall 66 are formed on the metal interconnection 32adjacent to the MTJ 62, in which each of the first slanted sidewall 64and the second slanted sidewall 66 could further include a curve (orcurved surface) or a planar surface.

Next, as shown in FIG. 5, a liner (not shown) is formed on the MTJ 62 tocover the surface of the IMD layer 30. In this embodiment, the liner ispreferably made of silicon oxide, but could also be made of otherdielectric material including but not limited to for example siliconoxide, silicon oxynitride, or silicon carbon nitride. Next, an etchingprocess is conducted to remove part of the liner to form a spacer 70adjacent to the MTJ 62 and another spacer 68 adjacent to the alignmentmark 56, in which the spacer 70 is disposed on the sidewalls of the MTJ62 and at the same time covering and contacting the first slantedsidewalls 64 and second slanted sidewalls 66 of the metalinterconnection 32 directly.

Next, as shown in FIG. 6, another IMD layer 72 is formed on the MTJregion 14 and logic region 16, and a planarizing process such as CMP isconducted to remove part of the IMD layer 72 so that the top surface ofthe IMD layer 72 is even with the top surface of the MTJ 62. Next, apattern transfer process is conducted by using a patterned mask (notshown) to remove part of the IMD layer 72 on the logic region 16 to forma contact hole (not shown) exposing the metal interconnection 26underneath and metals are deposited into the contact hole afterwards.For instance, a barrier layer 34 selected from the group consisting oftitanium (Ti), titanium nitride (TiN), tantalum (Ta), and tantalumnitride (TaN) and metal layer 36 selected from the group consisting oftungsten (W), copper (Cu), aluminum (Al), titanium aluminide (TiAl), andcobalt tungsten phosphide (CoWP) could be deposited into the contactholes, and a planarizing process such as CMP could be conducted toremove part of the metals including the aforementioned barrier layer 34and metal layer 36 to form a contact plug 74 in the contact holeelectrically connecting the metal interconnection 26. This completes thefabrication of a semiconductor device according to an embodiment of thepresent invention.

Referring again to FIG. 6, which further illustrates a structural viewof a semiconductor device according to an embodiment of the presentinvention. As shown in FIG. 6, the semiconductor device includes a MTJ62 disposed on the substrate 12, in which the MTJ 62 further includes abottom electrode layer 44, a pinned layer 46 disposed on the bottomelectrode layer 44, a barrier layer 48 disposed on the pinned layer 46,a free layer 50 disposed on the barrier layer 48, and a top electrodelayer 52 disposed on the free layer 50.

In this embodiment, the bottom electrode layer 44 preferably includes aconcentration gradient or more specifically that the concentrationdistribution of the bottom electrode layer 44 could gradually increaseupward, decrease upward, increase downward, or decrease downwarddepending on the material being used. In this embodiment the bottomelectrode layer 44 is preferably made of TaN and in such instance, theconcentration of nitrogen preferably increases from a bottom surface ofthe bottom electrode layer 44 to a top surface of the bottom electrodelayer 44 while the concentration of tantalum decreases from a bottomsurface of the bottom electrode layer 44 to a top surface of the bottomelectrode layer 44. In other words, the region or area in the layer 44immediately adjacent to the bottom surface of the bottom electrode layer44 or immediate adjacent to the junction between the bottom electrodelayer 44 and the metal interconnection 32 preferably includes higherconcentration of tantalum atoms and/or lower concentration of nitrogenatoms, whereas the region or area in the layer 44 immediately adjacentto the top surface of the bottom electrode layer 44 preferably includeshigher concentration of nitrogen atoms and/or lower concentration oftantalum atoms. Specifically, a ratio of nitrogen to tantalum closest toor at a top surface of the bottom electrode layer 44 is between 1.53 to1.87 and a ratio of nitrogen to tantalum closest to or at a bottomsurface of the bottom electrode layer 44 is between 0.18 to 0.22.

The semiconductor device further includes an alignment mark 56 disposedadjacent to the MTJ 62, in which the alignment mark 56 includes a pinnedlayer 46 disposed within the IMD layer 30 and on the stop layer 28, abarrier layer 48 disposed on the pinned layer 46, a free layer 50disposed on the barrier layer 48, and a top electrode layer 52 disposedon the free layer 50. Preferably, each of the pinned layer 46, barrierlayer 48, and free layer 50 includes U-shaped cross-section and thebottom surface of the alignment mark 56 is even with the bottom surfaceof the IMD layer 30 and slightly lower than the bottom surface of theMTJ 62.

Referring to FIGS. 7-8, FIGS. 7-8 illustrate a method for fabricating asemiconductor device according to an embodiment of the presentinvention. As shown in FIG. 7, it would be desirable to conduct aphoto-etching process to pattern the top electrode layer 52, free layer50, barrier layer 48, pinned layer 46, and bottom electrode layer 44 toform the MTJ 62 as shown in FIG. 4, and at the same time remove the topelectrode layer 52, free layer 50, barrier layer 48, and pinned layer 46in the trench 42 or alignment mark 54 to expose the surface of the stoplayer 28 once more. Next, it would be desirable to form a liner (notshown) as shown in FIG. 5 on the MTJ 62 to cover the surface of the IMDlayer 30 and filling part of the trench 42, and then an etching processis conducted to remove part of the liner to form a spacer 70 adjacent tothe MTJ 62 and a spacer 68 in the trench 42 or more specifically on theinner sidewalls of the alignment mark 54.

Next, as shown in FIG. 8, another IMD layer 72 is formed on the MTJregion 14 and logic region 16 as previously shown in FIG. 6, aplanarizing process such as CMP is conducted so that the top surfaces ofthe IMD layer 72 and MTJ 62 are coplanar and at the same time formanother alignment mark 56 adjacent to the MTJ 62, in which the alignmentmark 56 preferably includes a trench 42 within the IMD layer 30 andspacers 68 disposed on two sidewalls of the trench 42. Next, a patterntransfer process along with metal interconnective process are conductedto form metal interconnections or contact plugs on the logic region 16to electrically connect the metal interconnections 26. This completesthe fabrication of a semiconductor device according to an embodiment ofthe present invention.

Typically before patterning a MTJ stack to form the MTJ 62, a trench 42serving as an alignment mark 54 is formed adjacent to a predeterminedspot of a MTJ after the bottom electrode layer of a MTJ is deposited toimprove the accuracy of the patterning process. Since the bottomelectrode layer is usually composed of a material having uniformconcentration thereby causing more difficulty for the alignment mark 54to align during the aligning process, the present invention preferablyadjusts the material composition of the bottom electrode layer 44 toprovide a bottom electrode layer 44 having gradient concentration ormore specifically a bottom electrode layer 44 having concentration ofnitrogen increase from a bottom surface of the bottom electrode layer 44toward a top surface of the bottom electrode layer 44 while theconcentration of tantalum decreases from a bottom surface of the bottomelectrode layer 44 toward a top surface of the bottom electrode layer 44as disclosed in the aforementioned embodiment. By adjusting theconcentration distribution of the bottom electrode layer 44 in thismanner, it would be desirable to improve the visibility during thefabrication of alignment mark substantially.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

1. A method for fabricating semiconductor device, comprising: forming aninter-metal dielectric (IMD) layer on a substrate; forming a metalinterconnection in the IMD layer; forming a bottom electrode layer onthe IMD layer, wherein the bottom electrode layer comprises a gradientconcentration; forming a free layer on the bottom electrode layer;forming a top electrode layer on the free layer; and patterning the topelectrode layer, the free layer, and the bottom electrode layer to forma magnetic tunneling junction (MTJ).
 2. The method of claim 1, furthercomprising: forming a patterned mask on the bottom electrode layer;removing part of the bottom electrode layer and part of the IMD layer toform a trench; forming the free layer on the bottom electrode layer andinto the trench; forming the top electrode layer on the free layer; andpatterning the top electrode layer, the free layer, and the bottomelectrode layer to form the MTJ and an alignment mark adjacent to theMTJ.
 3. The method of claim 1, further comprising: forming a pinnedlayer on the bottom electrode layer; and forming a barrier layer on thepinned layer before forming the free layer.
 4. The method of claim 1,wherein the bottom electrode layer comprises tantalum nitride (TaN). 5.The method of claim 4, wherein a concentration of nitrogen increasesfrom a bottom surface of the bottom electrode layer to a top surface ofthe bottom electrode layer.
 6. The method of claim 4, wherein aconcentration of tantalum decreases from a bottom surface of the bottomelectrode layer to a top surface of the bottom electrode layer.
 7. Themethod of claim 4, wherein a ratio of nitrogen to tantalum at a topsurface of the bottom electrode layer is between 1.53 to 1.87.
 8. Themethod of claim 4, wherein a ratio of nitrogen to tantalum at a bottomsurface of the bottom electrode layer is between 0.18 to 0.22.
 9. Asemiconductor device, comprising: a magnetic tunneling junction (MTJ) ona substrate, wherein the MTJ comprises: a bottom electrode layer,wherein the bottom electrode layer comprises a gradient concentration; afree layer on the bottom electrode layer; and a top electrode layer onthe free layer.
 10. The semiconductor device of claim 9, furthercomprising: a pinned layer on the bottom electrode layer; and a barrierlayer on the pinned layer.
 11. The semiconductor device of claim 9,wherein the bottom electrode layer comprises tantalum nitride (TaN). 12.The semiconductor device of claim 11, wherein a concentration ofnitrogen increases from a bottom surface of the bottom electrode layerto a top surface of the bottom electrode layer.
 13. The semiconductordevice of claim 11, wherein a concentration of tantalum decreases from abottom surface of the bottom electrode layer to a top surface of thebottom electrode layer.
 14. The semiconductor device of claim 11,wherein a ratio of nitrogen to tantalum at a top surface of the bottomelectrode layer is between 1.53 to 1.87.
 15. The semiconductor device ofclaim 11, wherein a ratio of nitrogen to tantalum at a bottom surface ofthe bottom electrode layer is between 0.18 to 0.22.